1. Field of the Invention
The invention relates to intraluminal medical devices. More particularly, the invention relates to stents having nested interlocking segments that provide increased stability by remaining interlocked during delivery, and that provide enhanced vascular support, while maximizing fatigue durability of the implant.
2. Related Art
Percutaneous transluminal angioplasty (PTA) is a therapeutic medical procedure used to increase blood flow through an artery. In this procedure, an angioplasty balloon is inflated within the stenosed vessel, or body passageway, in order to shear and disrupt the wall components of the vessel to obtain an enlarged lumen. A dissection “flap” of underlying tissue can occur, however, which can undesirably fold into and close off the lumen. Immediate corrective surgery becomes necessary as a result.
More recently, transluminal prosthesis, such as stents, have been used for implantation in blood vessels, biliary ducts, or other similar organs of a patient in order to open, dilate or maintain the patency thereof. An example of such a stent is given in U.S. Pat. No. 4,733,665 to Palmaz. Such stents are often referred to as balloon expandable stents. A balloon expandable stent is typically made from a solid tube of stainless steel having a series of cuts made therein. The stent has a first smaller diameter, permitting the stent to be crimped onto a balloon catheter for delivery through the human vasculature to an intended treatment site. The stent also has a second, expanded diameter, that is achieved by the application of a radially, outward directed force by the balloon catheter from the interior of the tubular shaped stent when located at the intended treatment site.
Such balloon stents are often impractical for use in some vessels, such as the carotid artery. The carotid artery is easily accessible and close to the surface of a patient's skin. Thus, emplacement of a balloon expandable stent in such a vessel poses severy injury risks to a patient through even day-to-day activities, particularly where a force to the patient's neck could result in collapse of the stent within the vessel. Self-epanding stents have thus been devised in part to address these risks, wherein the self-expanding stent will recover its expanded state after being temporarily crushed by a force applied to a patient's neck or the like.
One type of self-expanding stent is disclosed in U.S. Pat. No. 4,655,771. The stent disclosed in U.S. Pat. No. 4,655,771 has a radially and axially flexible, elastic tubular body with a pre-determined diameter that is variable under axial movement of the ends of the body relative to each other and which is composed of a plurality of individually rigid but flexible and elastic thread elements defining a radially self-expanding helix. This type of stent is known in the art as a “braided stent” and is so designated herein. Placement of such braided stents in a body vessel can be achieved by a device which comprises an outer catheter for holding the stent at its distal end, and an inner piston which pushes the stent forward once it is in position.
Braided stents have many disadvantages, however, including insufficient radial strength to effectively hold open a diseased vessel. In addition, the plurality of wires or fibers comprising a braided stent become dangerous if separated from the body of the stent as they could pierce through the vessel. Tube-cut stents made from alloys having shape memory and/or superelastic characteristics have thus been developed to address some of the concerns posed by braided stents.
The shape memory characteristics allow the devices to be deformed to facilitate insertion into a body lumen or cavity, whereafter resumption of the original form of the stent occurs when subjected to sufficient heat from the patient's body, for example. Superelastic characteristics, on the other hand, generally allow the stent to be deformed and restrained in the deformed condition to facilitate insertion of the stent into the patient's body, wherein the deformation of the stent causes a phase transformation in the materials comprising the stent. Once within the body lumen of the patient, the restraint on the superelastic stent is removed and the superelastic stent returns to its original un-deformed state.
Alloys having shape memory/superelastic characteristics generally have at least two phases. These phases are a martensite phase, which has a relatively low tensile strength and which is stable at relatively low temperatures, and an austentite phase, which has a relatively high tensile strength and which is stable at temperatures higher than the martensite phase.
Shape memory characteristics are imparted to an alloy by heating the alloy to a temperature above which the transformation from the martensite phase to the austenite phase is complete, i.e., a temperature above which the austenite phase is stable (the Af temperature). The shape of the metal during this heat treatment is the shape “remembered”. The heat-treated alloy is cooled to a temperature at which the martensite phase is stable, causing the austenite phase to transform to the martensite phase. The alloy in the martensite phase is then plastically deformed, e.g., to facilitate the entry thereof into a patient's body. Subsequent heating of the deformed martensite phase to a temperature above the martensite to austenite transformation temperature causes the deformed martensite phase to transform to the austenite phase, and during this phase transformation the alloy reverts back to its original shape if unrestrained. If restrained, the metal will remain martensitic until the restraint is removed.
Methods of using the shape memory characteristics of these alloys in medical devices intended to be placed within a patient's body present operational difficulties. For example, with shape memory alloys having a stable martensite temperature below body temperature, it is frequently difficult to maintain the temperature of the medical device containing such an alloy sufficiently below body temperature to prevent the transformation of the martensite phase to the austenite phase when the device was being inserted into a patient's body. With intravascular devices formed of shape memory alloys having martensite-to-austenite transformation temperatures well above body temperature, the devices can be introduced into a patient's body with little or no problem, but they must be heated to the martensite-to-austenite transformation temperature which is frequently high enough to cause tissue damage.
When stress is applied to a specimen of an alloy or metal such as Nitinol exhibiting superelastic characteristics at a temperature above which the austenite is stable (i.e., the temperature at which the transformation of martensite phase to the austenite phase is complete), the specimen deforms elastically until it reaches a particular stress level where the alloy then undergoes a stress-induced phase transformation from the austenite phase to the martensite phase. As the phase transformation proceeds, the alloy undergoes significant increases in strain, but with little or no corresponding increases in stress. The strain increases while the stress remains essentially constant until the transformation of the austenite phase to the martensite phase is complete. Thereafter, further increases in stress are necessary to cause further deformation. The martensitic alloy or metal first deforms elastically upon the application of additional stress and then plastically with permanent residual deformation.
If the load on the specimen is removed before any permanent deformation has occurred, the martensitic specimen will elastically recover and transform back to the austenite phase. The reduction in stress first causes a decrease in strain. As stress reduction reaches the level at which the martensite phase transforms back into the austenite phase, the stress level in the specimen will remain essentially constant (but substantially less than the constant stress level at which the austenite transforms to the martensite) until the transformation back to the austenite phase is complete, i.e., there is significant recovery in strain with only negligible corresponding stress reduction. After the transformation back to the austenite phase is complete, further stress reduction results in elastic strain reduction. This ability to incur significant strain at relatively constant stress upon the application of a load, and to recover from the deformation upon the removal of the load, is commonly referred to as superelasticity or pseudoelasticity. It is this property of the material which makes it useful in manufacturing tube cut self-expanding stents.
The compressive forces associated with stent loading and deployment can pose concerns with respect to self-expanding stents. In stent designs having periodically positioned bridges, for example, the resulting gaps between unconnected loops may be disadvantageous. In both the loading and the deployment thereof, the stent is constrained to a small diameter and subjected to high compressive axial forces. These forces are transmitted axially through the stent by the connecting bridges and may cause undesirable buckling or compression of the adjacent loops in the areas where the loops are not connected by bridges.
Other concerns with self-expanding stents include reduced radiopacity, often resulting in the attachment of markers to the stent. The attached markers tend to increase the profile of the stent, and can dislodge from the stent or otherwise compromise the performance of the stent.
A still further concern is the transmission of forces between interconnected elements of a stent. Conventional vascular stents tend to comprise a series of ring-like radially expandable structural members that are axially connected by bridging elements. When a stent is subjected to in vivo bending, stretching or compression, its ring-like structural members distribute themselves accordingly, thus allowing the structure to conform to its vascular surroundings. These loading conditions cause the ring-like structural members to change their relative axial positions. The bridging elements help to constrain the ring-like structural members and therefore propagate strain between the ring-like structural members. The axial and radial expansion of the otherwise constrained stent, and the bending of the stent, that occurs during delivery and deployment, often renders conventional interconnected stents susceptible to fatigue fractures. Physiologic dynamics within the body of a patient also contribute to fatigue fractures of conventional stents.
Even where connected strut segments have been designed to disconnect upon deployment in order to minimize the occurrence of fatigue fractures, such as in co-pending U.S. patent application Ser. No. 10/687,143, filed Oct. 18, 2003, of common assignment herewith, such stents can prove unstable and susceptible to tipping or rotation within a vessel, particularly during delivery. Moreover, where the L/D ratio, i.e., the ratio of a expanded strut length L to an expanded diameter D, is greater than one due to a length L greater than a diameter D, for example, then the stent tends to be weaker and intended vessel support tends to be compromised. The weaker stent can be even more susceptible to fatigue fractures or other strain induced irregularities, while the longer stent segment lengths L result in larger gaps between structural components of the stent, which compromises vessel support. On the other hand, where the L/D ratio approaches zero, particularly where L approaches zero, then uniform and predictable positioning of the various segments comprising a stent is compromised. For example, where the length L of a protrusion approaches zero, then segments tend to de-couple before becoming firmly opposed to the lumen of the intended blood vessel. As a result, unpredictable propelling of the segments from the delivery device can occur.
In view of the above, a need exists for a stent having interlocked strut segments that remain connected during delivery until after deployment is effected so as to provide a more stable emplacement of the stent within a vessel or other body passageway. A need further exists to provide a stent having improved vessel support while minimizing fatigue fracture tendencies or other strain induced irregularities of the stent during loading, delivery and deployment thereof.